1. Field of the Invention
This invention relates generally to the formation of a solid article from a mathematical or graphical representation and more particularly to a system for generating the shape of an article by actinic irradiation of successive sheets which comprise a matrix which supports the article during formation.
2. Related History
Since the implementation of computer technology for physical design purposes, there has been an appreciation of the need for expanding use of computers to control the fabrication of prototypes of articles which have been designed. While the principle of irradiating an article to effect changes in molecular structure and physical properties has been long recognized, such phenomenon had not been employed for the purpose of generating three-dimensional articles until recent years. In this regard, attempts have been made to utilize a fluid medium containing a curable material which was irradiated by a plurality of beams. The energy level at the intersection of the beams was sufficient to cure the material within the fluid medium. Such apparatus did not gain widespread acceptance due to many inherent disadvantages including cost, resolution difficulties and insufficient energy levels as a result of absorption as the depth of beam penetration into the fluid medium increased.
In U.S. Pat. No. 4,575,330, an apparatus for constructing an article by ultra violet polymerization of the upper surface of a photopolymeric fluid medium was disclosed. Successive layers of the article were generated by successively lowering built up polymerized layers into the fluid medium so that the upper surface of the fluid medium contained unpolymerized material suitable for polymerization and generation of the next layer.
While the apparatus did not require multiple beams, it suffered from several disadvantages, including restricted photopolymeric material requirements. It was limited to use with ultra violet beams and required photopolymers responsive to the ultra violet wavelengths. In addition, difficulties were encountered in maintaining the upper surface of the fluid medium at a planar reference level due to fluid turbulence, reduction in fluid level as a result of the polymerization of prior layers and uneven shrinkage of the polymerized material. Also, the apparatus lacked precise control over the depth of polymerization into the fluid medium which resulted in loss of axial resolution.
Further, because a fluid medium was used, the article being constructed could not be supported except by the underlying built up article layers and a support platform, movable within the medium, was required. As such, articles having surfaces contoured with undercuts and the like could not be generated without providing a complicated mechanism capable of moving the platform aloha multiple axes.
In addition, in order to provide sufficient energy levels for curing, the maximum scan speed of the ultra violet beam was relatively slow, resulting in a correspondingly slow overall throughput.
In copending application Ser. No. 07/312,613 filed Feb. 17, 1989, an article shaping system was described wherein sheets of thermoplastic polymer mixture were stacked on a platen and the uppermost sheet was irradiated with an electron beam deflected to trace the configuration of a incremental planar heighth of the article. The beam effected cross-linking of the polymer along the trace to convert, for example, a thermoplastic polymer to a thermoset. As successive sheets were placed on top of one another and irradiated, the stack formed a matrix which supported the built up configuration of the article. The matrix was thereafter removed by heat and/or solvents. Such system provided enhanced image resolution, reduced mechanical complexity and greater material compatibility than prior methods. Throughput was increased because the electron beam provided an efficient irradiation source and the sheets could be stacked and referenced rapidly.
Unfortunately, the production of large and detailed components could still take many hours. Further, the efficient production of a suitable electron beam required expensive evacuation hardware systems and high voltage biasing equipment. Additionally, image penetration into underlying sheets presented a problem which could be alleviated by precise control of the beam energy level or the use of sheets of polymer mixture with depthwise varying degrees of reactivity.